This invention relates to a method and means for achieving receiver word alignment in digital transmission systems using redundant ternary line codes.
The use of redundant ternary line codes for digital transmission systems is well known. For example U.S. Pat. No. 3,611,141, whose disclosure is incorporated herein by reference, discloses a terminal, for a data transmission system, including a transmitter and a receiver, the transmitter having means for converting groups of binary digit signals into ternary digit signal groups each having either zero disparity or a disparity of one polarity only, means for inverting selected ternary groups having disparity of the one polarity into ternary groups having disparity of the opposite polarity, means for determining the cumulative disparity of the ternary signal output of the inverter and means for inhibiting the action of the inverter when the cumulative disparity of the ternary signal output of the terminal is of the opposite polarity, the receiver having means for converting zero disparity ternary groups into groups of binary digits, means for converting ternary groups with disparity of one polarity into binary groups and means for converting ternary groups with disparity of the opposite polarity into binary groups corresponding to ternary groups of the one polarity disparity which are the inverse of the ternary groups with the opposite polarity disparity. It is possible to determine misalignment in systems using such redundant codes by virtue of the fact that a misaligned receiver will apparently be receiving ternary words containing an excessive number of violations of the coding rules. Furthermore, any such misaligned receiver will apparently be receiving ternary words containing an excessive number of violations of the coding rules and, additionally, any such misaligned receiver will apparently be receiving a number of prohibited words, i.e. ternary words which do not have binary code group equivalents.